Project Summary Circadian rhythmicity is a fundamental aspect of human physiology. Proper circadian rhythms in cardiovascular output, metabolism, and sleep, for example, are essential to physical and psychiatric health, and the incidence of stroke, asthma, heart attack, and sleep disturbances all correlate strongly with time of day. However, understanding how circadian modulation of the body's physiology contributes to human health and disease will require a basic understanding of how circadian rhythms are generated and expressed. The goal of this proposal is to identify the basic mechanisms that drive circadian output from the suprachiasmatic nucleus (SCN), the brain's clock. SCN neurons demonstrate circadian rhythms in spontaneous action potential firing, and this rhythmic output is essential for timing circadian behaviors and physiology. Therefore, daily modulation of excitability is a critical basis for generating the circadian rhythm in neuronal activity in the SCN. The large conductance Ca2+-activated K+ channel (BK), encoded by the Kcnma1 gene, is an essential regulator of circadian physiological and behavioral rhythms in mouse and fly. In the SCN, BK currents shape circadian rhythms in neuronal firing by regulating the day-night difference in firing frequency, but the basic mechanisms underlying BK's actions are unknown. In the proposed research, we test the central hypothesis that the daily modulation of BK current properties drives the circadian rhythm in SCN neuronal firing. This hypothesis will be investigated using electrophysiology and molecular biology through the following specific aims: 1. To determine whether the constituents of the BK channel complex vary over the circadian cycle, 2. To determine how the properties of native BK currents vary with circadian phase in the SCN, 3. To test whether daily modulation of BK currents drives circadian rhythms in neuronal firing in the SCN. The significance of this research is that it provides a unique approach to directly link changes in the biophysical properties of an ionic current to changes in neural coding. The outcome of these studies is an understanding of the detailed mechanisms that govern circadian output. Understanding output from the brain's master clock is a critical step toward the development of novel ways to manipulate the circadian aspects of human physiology and pathophysiology.